Fluid migration in oil or gas wells is generally referred to as “casing vent flow” (CVF) or “gas migration” (GM) and can refer to any one or more of the following phenomena:                Fluid flowing from the formation into an outermost annular portion of the wellbore behind an outermost casing string in the wellbore;        Fluid flowing from the outermost annular portion of the wellbore into the formation; and        Fluid flowing across any of the casing or tubing strings in the wellbore.        
Fluid includes gas or liquid hydrocarbons, including oil, as well as water, steam, or a combination thereof. Any fluid migration will produce an “acoustic signal”. Acoustic signals resulting from the migration of fluid may be used as an identifier, or “diagnostic”, of a leaking well. For example, gas may migrate as a bubble from the source up towards the surface, frequently taking a convoluted path that may progress into and/or out of the production casing, the surrounding earth strata and the cement casing of the wellbore, and may exit into the atmosphere through a vent in the well, or through the ground. As the bubble migrates, pressure may change and the bubble may expand or contract and may increase or decrease its rate of migration and produce an acoustic event.
Fibre optic cables, acoustic sensor arrays and other acoustic sensing tools can be deployed downhole for detecting acoustic signals and locating fluids leaking from the wellbore. The acoustic sensing tools capture and detect acoustic signals and translate these signals to optical energy to provide a well profile. The acoustic data can be digitally processed by software algorithms to determine the origin of different sounds in the well and separate nuisance data from useful data allowing the location of the leak to be pinpointed.
Loud sounds can be caused for example by well surface equipment, the flow of water in an underground stream located above or below the tool location, a loud gas leak above or below the tool location, etc. These loud sounds can often overshadow sounds generated near the tool location to the point where the software algorithms may be incapable of detecting the presence of a sound event of interest.